You can get just the .50 brushes from Amazon, and probably elsewhere, but didn't look. They fit on most generic cleaning rods. They are only a few bucks each, vs a whole new .50 specific kit.
Aside from a bore brush also get a larger brush for the inside of the action. IIRC its about a 12 gauge size brush. Nylon will be fine for a action brush since the fouling should not be baked on as bad as a bore.